In the satellite digital television broadcasting and the ground-wave digital television broadcasting which is going to start soon, a transport stream (will be referred to simply as “TS” hereinafter) conforming to the MPEG (Moving Pictures coding Experts Group) 2 systems (ISO/IEC 13818-1) (will be referred to as “MPEG-2” hereinafter), is used as the broadcast wave. TS as the broadcast wave has multiplexed therein a video stream, audio stream and other data stream.
When TS as the digital television broadcast wave is recorded into a recording medium, a bit rate changer as shown in FIG. 1 is used to reduce the code bit rate for a to-be-recorded TS if the data recording rate into the recording medium is lower than at a bit rate at which the TS is to be recorded. Note that since the bit rate changer functions to convert the code bit rate, it is a kind of transcoder for converting codes of an input stream.
A TS demultiplexer 1 included in the bit rate changer shown in FIG. 1 separates an input TS via an input terminal IN into a video stream VS and audio stream AS and supplies them to an MPEG video decoder 2 and MPEG audio decoder 4, respectively.
The MPEG video decoder 2 decodes the supplied video stream VS to provide a video signal and supplies the video signal to an MPEG video encoder 3. The MPEG video encoder 3 will encode the supplied video signal to a video stream having a predetermined bit rate by the MPEG method, and supply the encoded video stream to a TS multiplexer 6. On the other hand, the MPEG audio decoder 4 decodes the supplied audio stream AS supplied to provide an audio signal and supplies the audio signal to an MPEG audio encoder 5. The MPEG audio encoder 5 will encode the supplied audio signal to an audio stream having a predetermined bi rate by the MPEG method, and supply the coded audio stream to the TS multiplexer 6. The TS multiplexer 6 will multiplex the video stream supplied from the MPEG video encoder 3 and the audio stream supplied from the MPEG audio encoder 5 to provide a TS′ having a reduced bit rate. The TS′ is delivered at an output terminal OUT.
However, the conventional bit rate changer shown in FIG. 1 is disadvantageous in that when an input TS contains any elementary streams not conforming to MPEG-2, the TS, once separated into elementary streams, cannot be re-multiplexed. For example, a private stream such as a data stream used in the digital television broadcasting is not conforming to MPEG-2, and each of service providers uniquely sets necessary buffer size and bit rate for multiplexing. No multiplexing is possible when the TS multiplexer 6 does not understand the buffer model for a stream to be multiplexed. Therefore, it is difficult to multiplex a private stream such as data stream for example except when the buffer model of the stream is already known.
Also, the conventional bit rate changer shown in FIG. 1 has a problem that it can change the bit rate of a transport stream TS having multiplexed therein one video stream VS and one audio stream AS, each conforming to MPEG-2 but cannot change the bit rate of a TS having multiplexed therein one video stream VS and a plurality of audio streams AS.